In recent years, the number of International Schools around the world has increased rapidly largely as a result of growth of new forms of International Schools, which differ markedly from the traditional forms. These new forms: are often operated on a for-profit commercial basis; are usually for children from the local (indigenous), wealthy population; and have been defined as International Schools because they are located in a non-English speaking country and English is the school's medium of communication. The growth of International Schools of the non-traditional type raises issues about the legitimacy of such schools. These new forms of International School face particular challenges in establishing themselves legitimately as "international". In this article, we develop a framework which is grounded in institutional theory to analyse the institutionalisation of and the consequent legitimacy of International Schools. We use the three pillars of institutionalisation which, by means of carriers, underpin the institutionalisation of organisations. We employ this framework to analyse and illustrate the legitimacy of a school's claim to be an International School and also to bring to light the challenges that schools face in establishing a legitimate claim to be "international".